This may be related to recent fixes of bug MDL-6386.

In 1.7.1, 1.8beta and 1.9dev (not tested in 1.6+)
I make a cloze question by using the code given in the online help:

This question consists of some text with an answer embedded right here

{1:MULTICHOICE:Wrong answer#Feedback for this wrong answer~Another wrong answer#Feedback for the other wrong answer~=Correct answer#Feedback for correct answer~%50%Answer that gives half the credit#Feedback for half credit answer}

and right after that you will have to deal with this short answer ... etc.

Then in question preview mode I click on the Fill with correct button.
All questions are filled with correct answer, except for MULTICHOICE questions.

Joseph